1. Field of the Invention
The present invention relates to an electronically controlled fuel supply method and a device for an internal combustion engine and particularly to a technique for optimizing a correction proportion of increasing correction of a fuel supply amount to an engine during cooling down according to properties of the used fuel.
2. Related Art of the Invention
In a conventional electronically controlled fuel supply device for an internal combustion engine, an air amount sucked into an engine is detected and a fuel supply amount according to this air amount sucked into the engine is calculated, while a fuel injection valve is driven and controlled according to this calculated fuel supply amount.
During cooling down, much of the fuel injected from the fuel injection valve adheres to the neighborhood of an intake valve, which decreases the fuel amount actually taken into a cylinder and makes the air/fuel ratio lean. Therefore, leaning of the air/fuel ratio during cooling down is prevented by increasingly correcting the fuel supply amount according to a cooling water temperature, representing the temperature of the engine, (See the Japanese Unexamined Utility Model Publication No. 62-162364).
The adhesion rate, the rate of fuel which adheres to the neighborhood of the intake valve, and the evaporation rate, the rate of the adhering fuel which evaporates and is taken into the cylinder, are different according to the properties of the fuel used at that time (especially the easiness of evaporation) even in the same temperature condition.
Therefore, in conventional examples, even when the fuel which is the hardest to evaporate was used, allowance was made and the proportion of the fuel increasing proportion according to the above mentioned water temperature was set larger so that leaning of the air/fuel ratio during cooling down which led to misfire or surge with the misfire should not occur.
Thus, when the fuel which is relatively easy to evaporate was used, the above fuel increasing correction became excessive and the air/fuel ratio was made overrich, which deteriorated fuel consumption and exhaust properties.
As a technique to compensate such nonconformance of the increasing correction due to difference in the used fuel, such a system was proposed that a sensor for detecting the fuel properties is provided and the increasing correction proportion according to the water temperature is optimized in conformance with the fuel properties detected by the above sensor (See the Japanese Unexamined Patent Publication No. 1-216040). However, as the above sensor for detecting the fuel properties is expensive, costs of the system were raised.